Need more useful warning messages for user error

[KedoKudo]

The measurement of the incident beam should not go through live reduction, which is safeguarded by a PV.
However, users, especially new ones, could forget to update the PV field during experiment, which results in the following error during processing:

`must be real number, not NoneType`

Here is one example case: https://monitor.sns.gov/report/ref_m/39493/

We need more meaningful error message from the reduction package so that the user is aware that they should check their experiment settings before moving onto the next measurement.